Bursts in the medial geniculate body: a comparison between anesthetized and unanesthetized states in guinea pig.

Thalamic high frequency bursts have long been described under anesthesia and during slow-wave sleep (SWS). More recently, studies in the lateral geniculate nucleus have pointed out that they are also present during waking (W). Here, we compared the bursts recorded in the medial geniculate body of guinea pigs under anesthesia or during periods of W and SWS. The tuning of single units was tested between threshold and 80 dB SPL in two conditions: (1) in restrained, undrugged, non-sleep-deprived guinea pigs (n=101 cells) and (2) under pentobarbital anesthesia (n=53 cells). Off-line analyses allowed us to distinguish single action potentials (APs) from bursts. A burst was defined as a group of APs with an interspike interval < or =4 ms, preceded by a silent period > or =100 ms. We found that auditory thalamus bursts occur in synchronized electroencephalogram states (SWS and anesthesia), but also during W. Although the burst characteristics did not differ among the three states, group data showed that the proportion of bursts within spike trains was the greatest under anesthesia. This observation resulted from two types of effects: (1) the percentage of non-bursting cells was lowest under anesthesia and (2) some cells under anesthesia exhibited up to 90% of bursts, whereas during W or SWS the highest proportion of bursts did not exceed 40%. The presence of these bursts is discussed with regards to the known fluctuations of membrane potential which occur in these various states.